The invention relates to a start switch for an AC motor, also called a disconnect or cut-out switch.
A single phase AC induction motor typically includes a squirrel cage rotor and two stator windings, namely a main winding and a start or auxiliary winding. The main winding is connected across an AC power source and normally has low resistance and high inductance. The start or auxiliary winding is physically displaced from the main winding and has high resistance and low inductance. In a split phase type AC motor, the physical displacement of the motor stator windings together with the electrical phase displacement produced by the different resistance and inductance values generates a rotating magnetic field sufficient to provide a low level starting torque. Other types of single phase AC induction motors for higher torque applications include capacitor start type motors and capacitor start run type motors. In these types of motors, a capacitor is connected in series with the start winding to provide higher starting torque. In each of the motor types, after the motor has accelerated to 75 to 80 percent of its synchronous speed, a switch opens and the start winding is disconnected from the AC power source.
Various types of disconnection systems are known. In one type of known system, the voltages across the main and start windings are compared, for example U.S. Pat. Nos. 4,622,506, 4,670,697, 4,782,278, including the relative phase therebetween, for example U.S. Pat. No. 5,559,418, and the start winding is disconnected in response to a given voltage magnitude and/or phase relation. In these types of systems, there is a comparison of voltage to voltage. In another system, for example U.S. Pat. No. 5,206,573, the start winding is disconnected in response to a given phase relation of the currents through the main and start windings. In these types of systems, there is a comparison of current to current.
The present invention provides a simple, low cost motor starting controller that is speed sensitive, load sensitive, and AC line voltage insensitive. The start winding is disconnected in response to a given phase relation between the main winding voltage and the start winding current. In this system, there is a comparison of voltage phase to current phase during starting of the motor. In the present system, there is provided a main winding voltage phase detector detecting the phase of voltage across the main winding, a start winding current phase detector detecting the phase of current through the start winding during the starting mode, and a cut-out circuit responsive to the main winding voltage phase detector and to the start winding current phase detector and actuating the start switch to the running mode, disconnecting the start winding from the AC source, in response to a given phase relation between the main winding voltage and the start winding current.